1. Field of the Invention
The present invention relates to a reference voltage generating circuit that generates a reference voltage with low temperature dependence.
2. Description of the Background Art
In order to improve the accuracy of a semiconductor device, fluctuations of a reference voltage caused by temperature change need to be very small. A bandgap reference (BGR) circuit is widely used as a circuit that generates such a reference voltage. The BGR circuit is generally configured to generate a reference voltage with low temperature dependence by adding a voltage having a positive temperature dependence and a voltage having a negative temperature dependence at an appropriate ratio.
In the actual BGR circuit, however, two input voltages of an operational amplifier as a component do not completely match with each other due to the element variation, and there is a voltage difference (hereinafter referred to as “offset voltage”) between the input voltages. Therefore, due to an influence of the offset voltage of the operational amplifier, the accuracy of the reference voltage decreases.
In order to eliminate the aforementioned influence of the offset voltage, U.S. Pat. No. 6,462,612, for example, proposes a BGR circuit having a chopper circuit incorporated thereinto. This BGR circuit converts an offset voltage component of an operational amplifier into an alternating current component by using the chopper circuit. Then, the BGR circuit removes this alternating current component by using a low pass filter (LPF) circuit, thereby generating an ideal reference voltage that does not include the offset voltage component.
In the technique described in the aforementioned document, an RC filter formed of a combination of a resistive element and a capacitive element is applied as the LPF circuit. The frequency characteristic of the RC filter is determined by selection of a resistance value of the resistive element and a capacitance value of the capacitive element.
On the other hand, the BGR circuit is a circuit that is widely used as a reference voltage source of the semiconductor device, and thus, low current consumption and small occupied area are required. In order to achieve low current consumption, the settling time of the operational amplifier cannot be shortened. Therefore, the frequency (chopper frequency) of a switch signal that controls the chopper circuit cannot be set high.
In order to remove the offset voltage component by using the low-frequency switch signal, the cutoff frequency of the LPF circuit needs to be set lower than the chopper frequency. However, as for the RC filter, at least one of the resistance value of the resistive element and the capacitance value of the capacitive element becomes larger as the cutoff frequency is reduced. Therefore, the area occupied by the LPF circuit becomes larger and the circuit scale of the BGR circuit increases. The other problems and novel features will become more apparent from the description of the specification and the accompanying drawings.